The present invention relates generally to digital printing systems and more particularly to an indirect digital printing system in which an ink image is formed on a printing member for subsequent transfer onto a printing substrate.
Two general types of printing systems are known in the art for printing digitally stored images, namely direct printing systems and indirect printing systems. In direct printing systems, one copy of the stored digital image is printed per one output of the digital data, the digital data being employed to control the flow of ink or other colorant, so as to form the printed image on the printing substrate. An example of a direct printing system is ink-jet printing systems.
Ink-jet printing systems in which ink is applied directly to the printed substrate have many deficiencies, inter alia, the following:
(1) Dust carried by the printing substrate and, when the substrate is paper, lint may also clog the orifices of the ink-jet nozzles. To minimize clogging, the substrate is placed at a relatively large distance from the nozzles, the large distance resulting in larger inaccuracy of the applied ink and leads to deficient quality of the printed image.
(2) For proper jetting, the ink must be of low viscosity, but this may cause undesirable spreading or penetration of the ink droplets on the medium and may also make the drying process lengthy or complex.
In indirect printing systems, on the other hand, the digital image is recorded on a printing plate, the printing plate includes a pattern of ink-receptive areas representing the recorded image within non-ink-receptive areas, and the printing process consists of applying ink to the entire surface of the printing form, wherein only the recorded image areas, i.e. the ink-receptive areas, retain the applied ink, and transferring the retained ink by contact to the printing substrate so as to form the printed image. The indirect printing process, carried out in a printing press, is classified according to the type of printing plate utilized, the three primary classes being salient, also known as letterpress or flexo; planographic, also known as lithographic or offset; and intaglio, also known as gravure.
Salient and planographic printing plates are usually configured as plates that are mounted around a cylinder in the printing press, while intaglio is usually formed on the surface of a solid cylinder.
The terms letter press, offset and gravure are used hereinbelow to indicate the three types of printing plates in any form, such as in the form of a plate or as a part of the printing cylinder.
In conventional printing presses, the image is recorded on the printing plate or cylinder off-press. In digital presses, in particular in digital offset presses, such as the Quick Master DI46-4, commercially available from Heidelberg Drachmachinen of Germany, the image is recorded on printing plate on-press, i.e. while the printing plate is mounted on the printing press.
In another printing system known in the ark described in U.S. Pat. No. 4,855,768 to Iino et al a printing member is formed from ink receptive dots forming a grid and from an ink repelling portion of ink repelling dots therebetween. Ink is applied onto the entire printing member and retained by all the ink receptive dots and then selectively transferred therefrom using electrostatic force so as to form the printed image.
Some embodiments of the present invention provide an indirect digital printing system and method for selectively applying ink on a printing member in accordance with the digital data representing the image to be printed so as to form an ink image thereon, the ink image subsequently transferred to a printing substrate, such as paper.
Other embodiments of the present invention are directed to an improved digital printing press, which employs an ink-jet printing unit for applying ink on a printing member so as to form an image ink thereon and a transfer system for transferring the ink image onto a printing substrate.
Still other embodiments of the present invention provide an indirect ink-jet printing system which employs an ink-jet printing unit for selectively applying ink on a printing member so as to form an ink image thereon and a transfer system for transferring the ink image onto a printing substrate.
Yet some embodiments of the present invention provide an ink-jet printing system having improved accuracy and resolution with respect to prior art ink-jet printing systems.
There is thus provided, according to some embodiments of the present invention, a printing system which includes at least one printing apparatus. Each printing apparatus includes a printing member for selectively receiving ink into an ink receptive portion, the ink receptive portion includes a plurality of ink receptive dots forming a pattern in a desired resolution and an ink repelling portion, the ink repelling portion includes the entire area of the printing member except the ink receptive dots. Each printing apparatus includes at least one printing unit, preferably but not necessarily an ink-jet printing unit, the printing unit for applying ink onto the printing member in accordance with a pattern representing an image to be printed, thereby forming an ink image on a portion of the ink receptive dots, and a transfer system for transferring the ink image onto a printing substrate.
Further, in accordance with some embodiments of the present invention, each printing apparatus also includes a cleaning system, the cleaning system for cleaning the printing member after operation of the transfer system.
Still further, the printing apparatus may include at least one heating system, the heating system for heating the printing member.
Additionally, where the ink used in the printing system is a UV curable ink, the printing system may further include a UV curing system downstream the at least one transfer system, the UV curing system for curing the UV curable ink transferred to the printing substrate.
In accordance with some embodiments of the present invention, each printing apparatus includes a plate cylinder, the plate cylinder for mounting the printing member thereon and the transfer system includes a blanket cylinder for transferring the applied ink to a printing substrate and an impression cylinder for impressing the printing substrate against the blanket cylinder.
According to alternative embodiments of the present invention, the printing member is a gravure like cylinder having depressions forming the ink receptive dots and elevated portions forming the ink repelling portion and the transfer system includes an impression cylinder for impressing the printing substrate against the gravure like cylinder.
According to yet another alternative embodiment of the present invention, the printing member is a cylinder having elevated portions forming the ink receptive portion and depressions forming the ink repelling portion. Further, the printing apparatus may include a blanket cylinder for transferring the applied ink to a printing substrate and an impression cylinder for impressing the printing substrate against the blanket cylinder. Alternatively, the transfer system includes an impression cylinder for impressing the printing substrate against the gravure like cylinder.
In accordance with some embodiments of the present invention, the printing member is a rotatable belt having elevated portions forming the ink receptive portion and depressions forming the ink repelling portion. Further, the transfer system includes a blanket cylinder for transferring the applied ink to a printing substrate and an impression cylinder for impressing the printing substrate against the blanket cylinder. Alternatively, the transfer system includes an impression cylinder for impressing the printing substrate against the rotatable belt.
Further, according to embodiments of the present invention, the at least one ink-jet unit includes a line array of ink-jet nozzles operating to apply the ink onto the printing member substantially simultaneously. The resolution of the ink receptive dots may be substantially similar to or higher than the resolution of the nozzles.
Still further, according embodiments of the present invention, the ink is an aqueous based ink, the ink receptive dots are hydrophilic and the ink repelling portion is hydrophobic. Alternatively, the ink is a non-aqueous based ink, the ink receptive dots are oleophilic and the ink repelling portion is oleophobic.
According to some embodiments of the present invention, the number of printing apparatus corresponds to the number of printing inks forming the ink image, Preferably, but not necessarily, the printing system includes four printing apparatus, each for inking one of the four process colors CMYK.
There is also provided, according to some embodiments of the present invention, a printing method which includes the following steps:
a. providing a printing member for selectively receiving ink onto an ink receptive portion, the ink receptive portion includes a plurality of ink receptive dots forming a pattern in a desired resolution; and an ink repelling portion, the ink repelling portion includes the entire area of the printing member except the in receptive dots;
b. applying ink onto the printing member in accordance with a pattern representing an image to be printed so as to form an ink image on a portion of the ink receptive dots; and
c. transferring the ink image onto a printing substrate.
Further, the method may also include cleaning the printing member after the transferring. Still further, the method may also include the step of heating the printing
According to a preferred embodiment of the present invention the ink is a UV curable ink and the printing method further includes curing the UV curable ink after the transferring.